In many fields of research and technology, optical imaging systems are used on which increasing demands for precision are made. The photolithographic methods used in the production of integrated semiconductor circuits are a particularly important example. The efforts to reduce the manufacturing costs of such circuits and to increase the switching speed lead on the one hand to a minimization of the circuits and consequently to their increased density on the semiconductor chip, and on the other hand to continuously increased dimensions of the semiconductor wafers on which a plurality of such semiconductor chips (chips) can be processed in one process step. At present the smallest circuit structures that can be made by means of photolithographic methods in the visible range of the spectrum are approximately 2 .mu.m. The semiconductor wafers, and consequently the field of view of the optical imaging systems used have reached a diameter of 10 to 15 cm. The high demands on the precision of the optical imaging systems involve even stricter demands on the precision of the methods applied for testing such imaging systems. If circuit structures with a minimum size of 1 .mu.m are to be made it is necessary to define the imaging characteristics of the photolithographic systems down to a precision of 0.1 .mu.m. These tests refer to the following parameters:
1. The local distortion of the imaging system at each point of the field of view. If the individual local distortion of each imaging system is known, systems with the same on similar local distortion characteristics can be selected and used together in one production line where several exposure steps with different systems are to be carried out. In this manner the overall yield, referring to the entire semiconductor wafer, can be increased considerably. PA1 2. Scale errors, i.e. deviations from the theoretical imaging scale of the imaging system (in photolithographic processes frequently 1:1). PA1 3. Further errors, as follow:
rotation, i.e. global rotations PA2 translation, i.e. global shifts PA2 orthogonality, i.e. angular distortions.
All these errors, while together represent the distortion, should be known individually for each point of the entire field of view. In the prior art, however, no methods are known by which such measurements can be executed with acceptable efforts and the necessary precision.
The testing process which is used most frequently today consists of imaging a specific test pattern, e.g. a vernier raster by the system to be tested, and measuring the image point by point, e.g. microscopically, in order to locate distortion errors. This method is, however, very time-consuming so that only a few points (e.g. 9) of the field of view can be measured. Furthermore, the precision that can be reached very much depends on the operator's practice and care (the respective points are individual points, and there is no automatic averaging over large local areas).
Apart from the measurement of individual points it is also possible to apply methods according to which large areas of the fields of view are tested in parallel for distortion errors. For that purpose, test patterns can be used which are imaged by the system to be tested, and subsequently tested for deviations. With test patterns having a periodic structure, the copy can be tested interferometrically, as shown in copending German patent application P 30 20 022.7, filed May 24, 1980 (U.S. counterpart application Ser. No. 266,243, filed May 22, 1981).
Another known method of comparison consists in the utilization of the moire effect obtained when the periodic original is superimposed with the copy. If the copy is imperfect owing to distortion, the moire stripes thus formed have a position which differs from that observed in the superposition of two ideal periodical structures. Details of the moire technique are described, e.g., in the article on p. 2455 of "Applied Optics", Vol. 11, 1972.
All hitherto known parallel measuring methods are restricted in their sensitivity to a value of approximately .gtoreq.0.5 .mu.m. Interferometric methods have the further disadvantages that they do not easily permit the investigation of the above described translation and rotation defects.
On the other hand, moire methods always permit the recording of all distortion errors. However, their sensitivity is in the order of the grating period used (approximately 0.1 grating period). The grating used for testing an optical system can only be made so fine that it can be resolved as a grating when imaged by the system to be tested. The smallest structures that can be resolved by photolithographic copies used today are approximately 2 .mu.m, so only those gratings can be used whose grating constant is higher than, or equal to 4 .mu.m. The possible sensitivity of these prior moire-metrical methods used with such gratings is thus limited to roughly 0.5 .mu.m, and thus they do not meet the above-mentioned requirements of a sensitivity of 0.1 .mu.m.
It is therefore an object of the present invention to provide a moire-metrical method for testing the imaging characteristics of optical imaging systems which permits testing of large fields of view in parallel with a precision of 0.1 micrometer.